Mark Levin likes to tell the story of how his Newbury Street venture capital firm, Third Rock Ventures, was born at a blackjack table in Las Vegas. He was playing at the Bellagio with two friends from his days as chief executive of Millennium Pharmaceuticals. All three were lamenting the timidity of pharmaceutical and venture capital firms when it came to putting money behind risky, but potentially high-impact scientific breakthroughs.

“What we were discussing was who would come along to fill that hole,” Levin says. “We all said, this is an interesting time to go out and grow great companies from the beginning.”

Since that night at the blackjack table, Third Rock has raised more than $800 million, and has been involved with starting or funding about 25 companies developing new drugs, medical devices, or diagnostic technologies. Third Rock is one of a trio of Back Bay firms that have been taking a much more hands-on approach to scouting for promising research and then crafting companies around it.

At a moment when money is flowing out of life sciences venture investing and many local venture capital firms have shrunk or closed up shop, Third Rock and its brethren are building companies that seek to improve the odds for in vitro fertilization, sequence the genomes of tumors to identify the best drugs to attack them, or regrow hair on balding pates. They’re more venture catalysts than venture capitalists.

Traditional venture capital firms in life sciences tend to take endless meetings with entrepreneurs who have ideas for new medical devices or who have found promising chemical compounds at a university lab that they hope to turn into a drug. When they find a deal they like, they hand over some money, often collected from a “syndicate” of fellow investors.

While firms like Third Rock, Longwood Fund, and PureTech Ventures sometimes do that, they’re more often reading journals and interviewing researchers to find promising scraps of science. Then, they assign SWAT teams of executives to try to build businesses around that germ of an idea. Often, they’re looking for areas that bigger pharmaceutical companies aren’t thinking about or researching. At Third Rock, the goal is to start five to seven new companies each year, Levin says.

One Third Rock company, Ember Therapeutics, aims to develop new drugs using an emerging understanding of the role brown fat plays in the body. Unlike ordinary white fat, brown fat burns, rather than stores, calories. To treat diseases like diabetes and obesity, new pharmaceuticals from Ember “could potentially turn current white fat into brown fat, or help the body produce new brown fat,” Levin says.

Last year, Ember raised $34 million from Third Rock and other investors. The company is run by Lou Tartaglia, a Third Rock partner who, like Levin, is an alumnus of Millennium Pharmaceuticals.

Another company, Foundation Medicine of Cambridge, enables doctors to send in a biopsy of a patient’s tumor and have its genetic make-up analyzed. “They can tell you what genes are mutated in this particular cancer, and what drugs are on the market, or in clinical trials, that a doctor might want to look at,” says Levin. “It gives you the chance to pick the right drug for the right person at the right time.”

The second of three venture capital firms, launched in 2010 by veterans of Sirtris Pharmaceuticals of Cambridge, is Longwood Fund. It has raised $85 million and formed two companies so far. One of them, OvaScience, is developing a treatment that would inject a fresh batch of mitochondria into a woman’s eggs to improve the odds that they’d become viable embryos once fertilized.

Another company, Verastem of Cambridge, was founded in 2010 and went public on the Nasdaq stock exchange earlier this year, raising $55 million. Run by Longwood cofounder Christoph Westphal, it is developing drugs that would kill cancer stem cells, which may be responsible for tumors that spring back after chemotherapy or radiation treatments.

Rich Aldrich, another Longwood Fund cofounder, notes, “There are fewer venture firms today actively putting money into early-stage start-ups,” but the result is there are “no feeding frenzies over new pieces of science like you would’ve seen six or eight years ago. If we see something we like out of academia, we can take some time to evaluate it.”

PureTech, located a few blocks down Boylston Street from Longwood, has spawned a much-watched start-up, Follica, which is working on a drug and medical device that would work together to stimulate hair growth. Gelesis, another PureTech company, is developing a capsule full of food-grade polymers that would expand in your stomach, taking up space and making you feel less inclined to eat that second whoopie pie.

Two newer PureTech projects focus on software, which can be far less expensive to develop than new drugs and medical devices. One, Knode, pores over patents and scientific literature to help find and rate experts on particular topics — something that could be useful to a pharmaceutical company looking for advice on whether to increase or decrease investment in a given field. Another, Akili Interactive Labs, is creating video games that it hopes could help alleviate depression or attention-deficit disorder.

Not all of these products will produce the results they need to win approval from the Food and Drug Administration or win over the doctors who must prescribe them. But having the deck stacked against you is what seems to motivate investors and entrepreneurs in the life sciences business.

I asked Aldrich whether it wouldn’t be easier to go develop a mobile app and hope that Facebook would acquire it for $1 billion. His response: “Once you’ve been involved in creating new medicines, other things don’t seem quite as important.”

Scott Kirsner can be reached at kirsner@pobox.com. Follow him on Twitter @ScottKirsner.

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.

Thanks & Welcome to Globe.com

You now have unlimited access for the next two weeks.

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.